This invention relates to an antenna system adapted to provide alignment of several antenna apertures in time to enhance the signal-to-noise ratio in antenna systems.
Steerable beam antenna systems typically consist of two basic types-reflector antennas and phased arrays. Although other antenna types exist, such as lens antennas, reflector and phased array antennas are by far the two most common.
Reflector antennas are simple and well understood and make up a significant portion of high gain antenna systems. In order to steer a reflector antenna, a mechanical movement of the entire antenna is usually necessary, although other means such as mechanical or electrical displacement of the feed have also been used. The structure which supports the reflector surface must provide certain precision to maximize the gain of the reflector. Surface deformation considerations can also cause the structural requirements to increase significantly as the size of the antenna increases.
In phased array antennas, the beam is steered electronically and the speed of beam motion is considerably faster than for a reflector antenna, especially for large regions of coverage. However, phased array antennas have several drawbacks. For example, they are typically much more expensive than reflector antennas, the signals sent and received at each element of the array must be phase and time aligned, and the gain of a phased array antenna decreases as the beam is steered off of the antenna boresight.
Current methods for automatic phase aligning a signal compare the phase of two signals using phase detectors, then adjust a phase adjuster associated with one of the signals until a phase difference is no longer detected. This phase alignment of signals from individual elements enhances the signal to noise ratio of the combined received signal from the array antenna. However, if the antenna is receiving a broadband signal, either a high data rate or composite multiple channel signal phase alignment may not result in an optimized signal to noise ratio, since phase alignment can occur in integer wavelength offsets. For relatively small phased array panels, phase alignment alone may be sufficient, as the distance between the center elements and edge elements may not be large enough to result in an integer wavelength offset between signals received at the elements. However, as the panel size in a phased array increases, the distance between the center and edge elements may be large enough to result in an integer wavelength offset between signals from the center elements and edge elements. Thus, for relatively large phased array panels, or widely spaced panels the signals also need to be aligned in time as well as phase to achieve an optimized signal-to-noise ratio for such a system. Signals which require both time and phase alignment to achieve an optimized signal to noise ratio are referred to as broadband signals. To compensate for this possibility, current methods for time aligning signals include using one or more reference signals at different frequencies to determine the required time offset. An external reference signal is sometimes used, which is a signal from a source external to the antenna apparatus which has a set, known frequency which is used to determine time offset between elements in the array. Alternatively, an internal reference signal may be used, which is a signal generated from within the antenna apparatus which is used to determine time offset between elements in the array.
Such a method is described in U.S. Pat. No. 5,041,836. In such a system an external beacon signal separate from the received signal is used to determine the amount of time adjustment required for each antenna element. The separate signals from the elements are first phase aligned, then the beacon signal is checked for phase alignment. The phase detector output will be proportional to the frequency ratio of the received signal and the beacon signal times the number of wavelengths of time delay difference in the received signal at the elements. While this system is successful in time aligning a broadband signal, it has drawbacks. For example, the maximum time delay error which can be detected is a function of the ratio of the frequency of the received signal and the frequency of the beacon signal. Thus, in an example shown in the above-mentioned patent, if the frequency of the received channel is chosen from 7.25-7.31 Ghz, and the beacon frequency is either 7.590 or 7.615 Ghz, the maximum time difference detectable is +/xe2x88x9211 wavelengths, and the maximum uncertainty in the absolute position of the elements must be within 18 inches. If larger time differences or uncertainties in position are required in an application, additional beacon frequencies may be used, or a larger difference in the received signal and beacon signal frequency can be used. If additional beacon signals are used, additional hardware is required, and if a larger difference in frequencies is used ambiguity may result in the smallest time delay bits. Thus, while allowing time alignment of broadband signals, this method requires additional hardware associated with the use of the one or more beacon frequencies, and is limited by ambiguity issues.
Digital hardware may also be used to determine required time offsets needed for each element of an antenna system. In such a case, a digital signal processor analyzes the signals from each element and determines the amount of phase and time shift for each element required to phase and time align all of the elements. In such a case, the digital processing hardware must be used, which can increase the cost of the system, and may also be limited by the signal processing capacity of the digital signal processor.
As mentioned above, the gain of a phased array decreases as the beam is steered off boresight. Due to this decrease in gain, phased array antennas typically are limited to scanning up to 60 conical degrees off the antenna boresight. Additionally, arrays are typically scaled to compensate for this scan loss by adding additional elements or amplifiers, which increases the cost of such an antenna. In order to increase the region of coverage beyond 60 degrees, often several apertures are used with each separate aperture including a separate phased array antenna. In such a case, the separate apertures are placed at angles to one another, with the signal being handed off from one aperture to an adjacent aperture when the scan angle to the first aperture becomes too large. The addition of other apertures allows scan angles beyond 60 degrees, with the signal typically being handed off between adjacent apertures at a scan angle to where the power level is equal between the adjacent faces. While this technique allows larger regions of coverage, several problems can be encountered when a beam is handed off between apertures. For example, phase coherency can be lost, bit synchronization can be lost, and there can be carrier and data drops during a signal handoff between apertures.
In accordance with the present invention, an antenna apparatus is disclosed that can determine phase and time delay between elements of a single phased array, or between apertures of a multiple aperture phased array antenna without the need for an independent external or internal reference signal. The phase and time delay can be determined using only a single received signal. Thus, there is no need for a separate beacon signal to be received at the antenna apparatus, nor is there a need to generate a separate reference signal within the antenna apparatus. The antenna apparatus includes an array of antenna elements for a single panel antenna, or multiple apertures in a multi-panel antenna. The elements or apertures are connected to at least a receive system which adjusts the received signal from each element or aperture to bring the signal into time and phase alignment. These same adjustments may then be used in a transmit mode to enhance a signal transmitted from the antenna apparatus.
The receive system includes a phase shifter or time delay circuit which is used to phase adjust the signal sent to and received from each element or aperture in order to obtain a phase aligned signal. This is done by analyzing a signal received at each element or aperture of the antenna apparatus. The signal received at a first element or aperture is selected as a reference signal. The signal received at a second element or aperture is then compared to the reference signal, and the signal associated with the second element or aperture is then adjusted based on a phase difference between the two signals. The signal received at each element or aperture is divided, with one portion of the divided signal routed to either an input of a correlator, for the reference signal, or to a switch, for the non-reference signals. The remaining portion of the divided signal is routed to a power combiner, which combines all of the signals. Once phase adjustment is complete for one element or aperture, the switch is set to select a signal from one of the remaining elements or apertures and the process is repeated for each non-reference element or aperture in the antenna, resulting in a combiner output which is an enhanced, phase aligned output signal. These same settings can then be used during the transmit mode to transmit an enhanced, phase aligned transmitted signal from the antenna. With proper design of the switches, the signal from any element can be the reference signal.
The adjustments to the signal associated with each element or aperture are made by analyzing the phase relationship between the reference signal and each non-reference signal and using the phase adjuster associated with each respective element or aperture to compensate for any phase differences between the signals. In determining the amount of phase adjustment to set for each element or aperture, the system uses a correlator which determines a phase delay to apply to each antenna element of the system in order to achieve an enhanced signal. The correlator operates by receiving the reference and non-reference signal at an input. The two signals are then mixed to create mixed channels within the correlator. The mixed channels are then analyzed to determine a phase relationship between the reference and non-reference signal. In one embodiment, the reference and non-reference signals are divided into two sub-signals each, with one of the non-reference sub-signals routed through a ninety degree phase shifter. The two reference sub-signals are mixed with the non-reference sub-signal and the phase-shifted non-reference sub-signal to create a zero degree channel and a ninety degree channel. The correlator outputs adjustment signals to control logic which then adjusts the phase shifter for the non-reference element or aperture based on the level of the signal in the mixed channels. Additionally, the ninety degree mixed channel may also be divided into two sub-channels, and one of the sub-channels inverted, creating ninety degree and negative ninety degree mixed channels. The comparator then analyzes the level of the signal in each of the zero, ninety degree and negative ninety degree channels to output a more accurate adjustment signal to the control logic which adjusts the phase shifter associated with the non-reference element or aperture.
In another aspect, broadband signals may be brought into time alignment. Dual correlators are used to represent two channels of correlated signals. The amplitude of the signals in the respective channels are then compared to each other with a comparator. The channel with the lower amplitude is then time adjusted to bring the broadband signal into time alignment, thus increasing the gain for the broadband signal. The comparison of each correlated channel is made by splitting each of the zero degree and ninety degree channels of each correlator. These channels are then squared and summed. The squared and summed channel from each correlator channel is then compared at the comparator, and a time adjustment is made to the reference and non-reference element or aperture based on the output of the comparator.
Based on the foregoing summary, a number of advantages of the present invention are noted. An antenna apparatus is provided that improves previously developed self-steered phased arrays by using a single signal to determine the time delay required to steer the antenna elements of the system in a broadband manner. Additionally, all of the components can be analog components, allowing the system to operate throughout a large range of frequencies. The analog correlator helps to compensate for errors in the position of the elements, the pointing direction, the path length from the target to the elements (including atmospheric effects), and the position of the target. Further, for multiple aperture antennas, this method and apparatus also allows for smaller, less expensive, apertures as each aperture does not need to be scaled or amplified to compensate for as much scan loss. An even further advantage is that the apertures need not be directly adjacent to one another, and may be located some distance apart.